理解Activity

Activity简单理解即是MVC中的C，即View Controller。以下是一段常见的Activity类的代码：

class MainActivity : AppCompatActivity() {

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)
		
		// ...
    }
}

理解setContentView()方法

函数定义：

public void setContentView (View view)

功能：

setContentView()用来为Activity设置其顶层视图，该视图可以包含其他子视图。

Set the activity content to an explicit view. This view is placed directly into the activity’s view hierarchy. It can itself be a complex view hierarchy. When calling this method, the layout parameters of the specified view are ignored. Both the width and the height of the view are set by default to ViewGroup.LayoutParams.MATCH_PARENT. To use your own layout parameters, invoke setContentView(android.view.View, android.view.ViewGroup.LayoutParams) instead.

理解LayoutInflater的用途以及如何用它创建视图

在Android中经常见到以下代码用于动态添加视图到现有视图中：

// Create a new view, which defines the UI of the list item
val view = LayoutInflater.from(viewGroup.context)
            .inflate(R.layout.text_row_item, viewGroup, false)
containerView.addView(view);

LayoutInflater的用途

Instantiates a layout XML file into its corresponding View objects. It is never used directly. Instead, use Activity.getLayoutInflater() or Context.getSystemService to retrieve a standard LayoutInflater instance that is already hooked up to the current context and correctly configured for the device you are running on.

For performance reasons, view inflation relies heavily on pre-processing of XML files that is done at build time. Therefore, it is not currently possible to use LayoutInflater with an XmlPullParser over a plain XML file at runtime; it only works with an XmlPullParser returned from a compiled resource (R.something file.)

How to inflat a view

  1. Get the LayoutInflater:
    Obtain a LayoutInflater instance from your context. This can be done using Activity.getLayoutInflater() or getSystemService(Context.LAYOUT_INFLATER_SERVICE).
  2. Inflate the Layout:
    Call the inflate() method of the LayoutInflater instance, providing the resource ID of the XML layout file and a ViewGroup (which will be the parent of the inflated view).
  3. Add the Inflated View:
    If you’re not inflating directly into a parent view (e.g., using attachToRoot = true), you’ll need to manually add the inflated View to its parent ViewGroup using addView().

接下来，让我们逐步理解前面代码用到的两个方法以及Context对象。

理解from()方法

函数定义：

public static LayoutInflater from (Context context)

功能：Obtains the LayoutInflater from the given context.

理解inflate()方法

函数定义：

public View inflate (int resource, 
                ViewGroup root)

public View inflate (int resource, 
                ViewGroup root, 
                boolean attachToRoot)

功能：Inflate a new view hierarchy from the specified xml resource. Throws InflateException if there is an error.

理解Context对象

Interface to global information about an application environment. This is an abstract class whose implementation is provided by the Android system. It allows access to application-specific resources and classes, as well as up-calls for application-level operations such as launching activities, broadcasting and receiving intents, etc.

在Android开发里Context参数很常见。使用Context参数，单例可完成启动activity、获取项目资源，查找应用的私有存储空间等任务。

注意，在get(Context)方法里，我们并没有直接将Context参数传给构造方法。该Context可能是一个Activity，也可能是另一个Context对象，如Service。在应用的整个生命周期里，我们无法保证需要用到Context时，Context就一定会存在。因此，为保证单例总是有Context可以使用，可调用getApplicationContext()方法，将不确定是否存在的Context替换成application context。application context是针对应用的全局性Context。任何时候，只要是应用层面的单例，就应该一直使用application context。

References:

  • https://developer.android.com/reference/android/view/LayoutInflater
  • https://book.douban.com/subject/25848404/
  • https://developer.android.com/reference/android/app/Activity#setContentView(android.view.View)
  • https://developer.android.com/guide/components/activities/intro-activities